We are investigating the methylation of the N Epsilon-amino groups of the lysyl residues in the arginine-rich histones in developing rat brain. The brain is extremely rich in histone lysine methyltransferase early in life. The transfer of methyl groups to the N epsilon-amino group of lysine is a late event occurring after histones are bound to DNA. In developing rat brain a fraction of the histone molecules bound to DNA have not been fully methylated, consequently chromatin from these animals can be utilized as substrate to investigate the kinetics of methylation in vitro. The enzyme catalyzing the methylation of the lysyl residues in histones H3 and H4 is bound to chromatin; however, it can be selectively eluted by sequentially extracting chromatin with water. We have purified the enzyme to homogeneity through gel filtration and chromatography on DEAE-cellulose and hydroxyapatite. Such properties as molecular weight, substrate specificity, number of subunits, Km value for S-adenosylmethionine and inhibitor constants for the D and L isomers of S-adenosylhomocysteine are being determined. The effects of pH, polyamines and various cations on the methylation of histones bound to DNA will be examined. We have found that that the concentration of S-adenosylhomocysteine and S-adenosylmethionine can be altered in the cell under certain conditions, ie. folic acid deficient diets and treatment with drugs including methotrexate or erthro-9-(2-hydroxy-3-nonyl)adenine. We are investigating the effects of shifts in the relative concentrations of adenosylhomocysteine and adenosylmethionine have on the rate and extent of methylation of chromosomal macromolecules, particularly histones.